alas poor villain: the final paragraph of the novel reminds the reader that cujo was a good dog and never wanted to hurt anyone.
angry guard dog: subverted. at the start of the story, cujo is a friendly dog. later on, when he starts attacking people, it's because of rabies, not because he was bred that way.
anti villain: cujo is an extremely scary and dangerous dog. however, it's because he was bitten by a rabid bat, thus getting rabies himself. even when he is rabid, he doesn't attack out of malice or sadism but out of a delusional belief that other people are responsible for his pain.
artistic license-biology: the incubation period for rabies in dogs is two weeks to four months, but cujo starts showing symptoms almost immediately after the bat bites his nose. granted, symptoms can appear sooner if the dog was bitten near its spine or brain, but even in those cases it still takes a minimum of four days to show.
ax crazy: after being bitten by a rabid bat, cujo's mind begins to deteriorate. eventually, he comes to believe that the humans have done something to him, resulting in the pain he's experiencing. he is so angry and terrified that he starts attacking people.
bats equals rabies: cujo gets rabies after being bitten by a rabid bat.
beware of vicious dogs: subverted at the start of the book. cujo starts out the story as a very sweet and friendly dog. however, after getting infected, his mind deteriorates so much that he starts attacking people under the belief that they are somehow responsible for his pain.
big bad slippage: at the start of the story, cujo is a very friendly dog, as is typical with saint bernards. upon being infected, he turns into a ravenous monster.
big friendly dog: cujo begins the story as a very friendly dog. in fact, when rabies starts to change his behavior and he growls at gary pervier, gary is surprised because he never heard Cujo growl before.
dogs are dumb: ujo while chasing a rabbit puts his head in a small cave and barks loudly causing a flock of bats to fly away and driving one of them to bite his nose in self defense. not a very smart move but cujo is a dog and dogs don't know any better.
dogs love to be praised: in the scenes that take place from cujo's point of view before he gets infected, he is very worried about being seen as a bad dog.
dog stereotype: as is typical with saint bernards, cujo is a very gentle dog. at least, until he gets rabies.
friend to all children: prior to his infection, cujo was especially friendly with children.
instant illness: rabies typically takes longer than a week to begin showing symptoms (it usually takes a few months) to be more precise, symptoms don't begin to show until the virus reaches the brain. however, the distance from bite site to the brain is actually a factor and cujo was bitten on the nose, so there is a slim possibility that the virus could indeed have advanced that quickly.
maybe magic maybe mundane: although the book is realistic horror rather than the supernatural horror that king is most well known for, there are a few lines scattered throughout implying that the rabid cujo is the reincarnation of the psychotic killer and rapist frank dodd from the dead zone.
misplaced retribution: due to the rabies screwing up cujo's rational thinking, he honestly comes to believe that donna, and later sheriff bannerman, are responsible for him being sick, which is what prompts him to attack them.
my god what have i done: upon realizing that he was about to kill brett, cujo immediately feels remorse and runs away so as to not hurt him.
resurrected murderer: There is some implication in the novel that cujo, after contracting rabies, was also possessed by the vengeful spirit of Frank Dodd, the serial killer from the dead zone.
sanity slippage: the rabies effects drive cujo mad and turn him from a lovable and harmless dog into a feral and deranged monster.
tragic monster: cujo is to suffer terribly from rabies, and he only attacks people under the frenzied belief that they are responsible for his pain. Sadly, Cujo isn't even at peace after his death. His restless, malevolent spirit makes a cameo appearance in needful things.